#[cfg(test)]
mod test {

    use std::collections::{btree_map::Keys, HashMap};

    #[test]
    fn test1() {
        let mut map = HashMap::new();
        map.insert("a", 1);
        map.insert("b", 2);
        map.insert("c", 3);
        let m = map.iter().min_by(|a, b| a.1.partial_cmp(b.1).unwrap());
        if let Some((k, v)) = m {
            println!("{}", k);
        }
    }

}